UC Riverside

Supported noble metal (e.g., Pt and Au) catalysts have been extensively studied in heterogeneous catalysis, since they are of great importance in chemical industries. It has been demonstrated that the metal-oxide interface can create new active sites, modify the electronic properties of the oxide, and further improve the reactivity and selectivity of catalytic reactions. Moreover, the metal catalysts are susceptible to irreversible deactivation through sintering, coking and leaching under reaction conditions. Therefore, we employed Atomic Layer Deposition (ALD) to prepare the TiO2 films as a protective layer to stabilize the catalysts and enhance the catalytic performance in the meantime.N2 adsorption-desorption isotherm analysis proved that the TiO2 films were grown in a layer-by-layer fashion, and the growth rate is estimated to be approximately 1 Å per cycle. X-ray photoelectron spectroscopy (XPS) results pointed that the titanium was fully oxidized. Infrared absorption spectra showed that new surface sites were created after the TiO2 deposition. Nuclear magnetic resonance (NMR) spectra suggested that the ALD process occurs preferentially at isolated silanol groups on SBA-15. Electron paramagnetic resonance (EPR), X-ray diffraction (XRD) and transmission electron microscopy (TEM) data indicated that the TiO2 films are amorphous and reducible. The TiO2 films and noble metal nanoparticles were deposited on SBA-15 in two possible sequences. TEM images confirmed that the Pt nanoparticles were uniformly dispersed in the pores of SBA-15. Infrared absorption spectra showed that active sites for both Pt and TiO2 are present. The nominal Pt loading (1 wt.%) is very close to the values measured by inductively coupled plasma-optical emission spectrometer (ICP-OES). The TiO2 loadings are in good agreement with TiO2/SBA-15 samples. The catalytic performance of supported Pt nanocatalysts was investigated by hydrogenation of cinnamaldehyde (CAL) and CO oxidation reactions. 1-TiO2/Pt/SBA-15 exhibits relatively the best conversion of CAL and selectivity of cinnamyl alcohol (COL), indicating that the addition of TiO2 films can improve the selectivity toward COL production. The enhanced catalytic performance is attributed to the creation of acidic sites and the changes of reactant adsorption modes. All the (1-4)-TiO2/Pt/SBA-15 and Pt/(1-4)-TiO2/SBA-15 samples show higher reactivity than Pt/SBA-15, suggesting the addition of TiO2 can promote CO oxidation. Oxygen vacancies created in the interface between the reducible TiO2 films and Pt nanoparticles is responsible for the enhanced reaction rate. The stability of the catalysts were investigated by running the hydrogenation reaction continuously for 3 times. The Pt nanoparticles were still well-dispersed in the pores of SBA-15, and no nanoparticle sintering was observed after 3 cycles of reaction in the TEM images. The enhanced stability is possibly due to (1) encapsulation of Pt nanoparticles inside SBA-15; (2) ALD-TiO2 protective layer; (3) SMSI effect. The supported Au nanocatalysts were synthesized using a gold cationic complex precursor-Au(en)2Cl3. Characterization results and kinetic data of hydrogenation of CAL reaction are similar to Pt catalysts. Oxidation of benzyl alcohol was studied for Au catalysts. Although the selectivity toward benzaldehyde was not greatly improved after the addition of TiO2, the stability of the Au/TiO2/SBA-15 and TiO2/Au/SBA-15 samples was still excellent.